The present invention relates to coherent energy beam imaging and, more particularly, to a novel method for improved adaptive formation of the coherent beam, using iterative phase conjugation, to counteract the effects of inhomogeneous wave propagation.
It is well known that phase cancellation effects, introduced by inhomogeneous wave propagation in a medium, limit both the spatial and contrast resolution of images derived from a beam of coherent energy, such as a beam of radar, sonar or ultrasound energy and the like. Phase cancellation effects are most acute for high frequency imaging using large apertures, (i.e. for large numerical apertures) where small arrival time differences over substantial distances result in large errors in the relative phase across that aperture. For example, in medical ultrasound this occurs because the phase of an interrogating ultrasound wave is distorted due to inhomogeneous distribution of sound velocities in the body through which the ultrasound wave propagates, from the aperture to the region of interest and the subsequent return to the aperture. It is well known that this effect can dominate the appearance of an ultrasound image as both the ultrasonic frequency and the size of the aperture are increased. While several methods have previously been proposed for reducing phase cancellation effects, all such methods operate, if at all, at the expense of spatial resolution. In order to image at the theoretical resolution of the aperture, especially at high frequencies, phase distortion of the incident ultrasound wave must be reduced. In ultrasound imaging, the predominant distortion occurs within the wall of the body being imaged, which can be modeled as a single distorting surface. For all regions of interest deeper than this body wall, it is highly desirable to reduce phase aberration by obtaining accurate information from which phase correction can be computed. This goal was attained using a method for the phase conjugation adaptive reduction of phase aberration effects upon the time delays necessary for formation of a beam of coherent energy focused within a non-homogeneous medium at a selected range R from, and at an angle .theta. with respect to the normal to, the surface of an array of a plurality N of transducers, each for providing a portion of the energy of the beam when excited and for converting energy reflected thereto to a signal therefrom, as described and claimed in copending U.S. application Ser. No. 099,422, filed Sept. 21, 1987, now U.S. Pat. No. 4,835,689, issued May 30, 1989, assigned to the assignee of the present invention; and incorporated herein in its entirety by reference. In that application, the method comprised the steps of bouncing a probe beam, derived from the entire array of transducers, from a large collection of scatterers, contained in a portion of the medium to be investigated; cross-correlating, for each probe beam, the received signals from each of the (N-1) pairs of adjacent transducers to derive a like number of phase conjugation correction signals; arithmetically operating upon the plurality of phase conjugation correction signals to provide a time correction for the time delay associated with each probe beam transducer, for that range R and angle .theta.; modifying by the time correction for that transducer the time delay used for formation of an actual imaging excitation beam; and then modifying by the time correction for that transducer the time delay of return signals, received from the media portion to be investigated, to reduce phase aberration in the resulting image. The resulting corrected image, while better than an uncorrected image, still leaves something to be desired in the way of further phase aberration reduction, especially where very large initial phase distortion is encountered.